In general, motorcycles are steered by operating a handlebar so as to turn a front fork supporting a front wheel around a steering stem. The steering stem is rotatably inserted into a head pipe located at the front end of a body frame. Such motorcycles are sometimes equipped with a steering damper device between a steering side member which is turned by operating the handlebar and a body side member which is not turned by operating the handlebar. In this case, the steering damper device requires at least the following conditions: damping moment is scarcely generated during normal traveling in which the steering angle of a handlebar is small and its angular speed is low; high-damping moment is generated in the range where the steering angle of the handlebar is large and the angular speed is high; and the damping moment is scarcely generated while the handlebar which has been turned is returned.
The present inventors have proposed a steering damper device that can meet such requirements using a cylindrical damper which is simple in configuration and low in cost (Japanese Patent Application No. 2003-301072). This steering damper device is configured such that the damper is composed of a damper casing and a damper rod, one of them is rotatably connected to a vehicle body side member such as a head pipe or the like, and the other is rotatably connected to a steering side member such as a fork bridge or the like. The damper is disposed so as to be most contracted or extended when the handlebar is in the neutral position, that is, when the steering angle of the handlebar is 0°.
With such a steering damper device, when the handlebar is turned to the right or left, the damper rod of the damper is slidably displaced in the damper casing in the same direction, which provides symmetrical damper-characteristics. When the handlebar is returned, the damper rod is slidably displaced in the opposite direction; therefore, a damping force can be allowed to be scarcely generated at that time. In addition, the amount of slidable displacement of the damper rod relative to the steering angle of the handlebar is small when the steering angle is in the vicinity of 0°; it is greater as the steering angle is larger than 0°. Likewise, the damping force applied to the steering side member from the damper is small when the steering angle of the handlebar is near 0°; it is greater as the steering angle is progressively changed from 0°. Thus, the damping force is scarcely generated during normal traveling in which the steering angle of the handlebar is small and the angular speed is low, and high-damping moment is generated in the range where the steering angle of the handlebar is large and the angular speed is high.